A diverse group of halogenated organic compounds (e.g., DDT, PCBs [polychlorinated biphenyls]) commands special attention in public health because they are ubiquitous in the ecosystem and may be causing adverse human health effects at background levels of exposure. We will conduct or are conducting several studies of the human health effects of background exposure to organochlorines. We have in the field now a case-control study of serum p,p'-DDE (a metabolite of DDT) and PCB levels in relation to risk of breast cancer among African-American women in Los Angeles. Three hundred cases and 300 controls will be enrolled, and the data should be ready for analysis in 1999. We are also planning to look at health effects of organochlorine residues among participants in the Collaborative Perinatal Project, for whom a blood bank (mother's blood) is available. Projects to be evaluated, and carried out, if appropriate are: 1) an examination of mother's serum p,p'-DDE levels in relation to risk of hypospadias and cryptorchidism in male offspring, 2) in utero PCB exposure in relation to hyporeflexia/hypotonia at birth, and 3) mother's serum p,p'-DDE levels in relation to subsequent sperm counts and fertility of male offspring.